We present a combined computational and experimental study to optimize the efficiency of evaporative cooling for atoms in optical dipole traps. By employing a kinetic model of evaporation, we provide a strategy for determining the optimal relation between atom temperature, trap depth, and average trap frequency during evaporation given experimental initial conditions. We then experimentally implement a highly efficient evaporation process in an optical dipole trap, showing excellent agreement between the theory and experiment. This method has allowed the creation of pure Bose-Einstein condensates of ${}^{87}$Rb with $2\times {10}^4$ atoms starting from only $5\times {10}^5$ atoms initially loaded in the optical dipole trap, achieving an evaporation efficiency ${\gamma }_{\mathrm{eff}}$ of 4.0 during evaporation.

We use collective oscillations of a two-component Bose-Einstein condensate (2CBEC) of ${}^{87}$Rb atoms prepared in the internal states $|1\rangle \equiv |F=1,m_F=-1\rangle$ and $|2\rangle \equiv |F=2,m_F=1\rangle$ for the precision measurement of the interspecies scattering length $a_{12}$ with a relative uncertainty of $1.6\times {10}^{-4}$. We show that in a cigar-shaped trap the three-dimensional (3D) dynamics of a component with a small relative population can be conveniently described by a one-dimensional (1D) Schrödinger equation for an effective harmonic oscillator. The frequency of the collective oscillations is defined by the axial trap frequency and the ratio $a_{12}/a_{11}$, where $a_{11}$ is the intraspecies scattering length of a highly populated component 1 and is largely decoupled from the scattering length $a_{22}$, the total atom number and loss terms. By fitting numerical simulations of the coupled Gross-Pitaevskii equations to the recorded temporal evolution of the axial width we obtain the value $a_{12}=98.006\left(16\right)a_0$, where $a_0$ is the Bohr radius. Our reported value is in reasonable agreement with the theoretical prediction $a_{12}=98.13\left(10\right)a_0$ but deviates significantly from the previously measured value $a_{12}=97.66a_0$ [Phys. Rev. Lett. 99, 190402 (2007)] which is commonly used in the characterization of spin dynamics in degenerate ${}^{87}$Rb atoms. Using Ramsey interferometry of the 2CBEC we measure the scattering length $a_{22}=95.44\left(7\right)a_0$ which also deviates from the previously reported value $a_{22}=95.0a_0$ [Phys. Rev. Lett. 99, 190402 (2007)]. We characterize two-body losses for component 2 and obtain the loss coefficients ${\gamma }_{12}=1.51\left(18\right)\times {10}^{-14}{\mathrm{cm}}^3/\mathrm{s}$ and ${\gamma }_{22}=8.1\left(3\right)\times {10}^{-14}{\mathrm{cm}}^3/\mathrm{s}$.

We show that metamaterials can be used as a testing ground to investigate spontaneous symmetry breaking associated with non-Hermitian quantum systems. By exploring the interplay between near-field dipolar coupling and material absorption or gain, we demonstrate various spontaneous breaking processes of the $\mathcal{PT}$ symmetry for a series of effective Hamiltonians associated with the scattering matrix. By tuning the coupling parameter, coherent perfect absorption, laser action, and gain-induced complete reflection ($\pi$ reflector) by using an ultrathin metamaterial can be obtained. Moreover, an ideal $\mathcal{PT}$ symmetry can be established effectively in a passive system by exploring the balance between scattering and absorption loss using metamaterials.

The photo-polaron in a two-dimensional optical microcavity is investigated within the framework of quantum statistical mechanics. Our experimental setup is based on a planar Fabry-Pérot cavity filled with a nonlinear nonpolar crystal. The Fabry-Pérot cavity provides a chemical potential and an effective mass for a photon, making the system formally equivalent to a two-dimensional gas of number-conserving, massive bosons. The incident photons interact with the Raman phonons in the crystal and hence are converted into quasiparticles, which we refer to as photo-polarons. A photo-polaron is a photon dressed with a cloud of virtual transverse-optical phonons, namely, the dressed photon. One of the most important properties of photo-polaron is an increased inert mass. Because of the effect of photo-polarons, we find that the photon-photon interaction is enhanced greatly. The photo-polaron is a quantum effect of cavity nonlinear optics but its increased inert mass is a classical property.

We address binary optical communication channels based on phase-shift-keyed coherent signals in the presence of phase diffusion. We prove theoretically and demonstrate experimentally that a discrimination strategy based on homodyne detection is robust against this kind of noise for any value of the channel energy. Moreover, we find that a homodyne receiver beats the performance of a Kennedy receiver as the signal energy increases and achieves the Helstrom bound in the limit of large noise.

The spontaneous nucleation and dynamics of topological kink defects have been studied in trapped arrays of 41–43 Yb ions. The number of kinks formed as a function of quench rate across the linear-zigzag transition is measured in the underdamped regime of the inhomogeneous Kibble-Zurek theory. The experimental results agree well with molecular dynamics simulations, which show how losses mask the intrinsic nucleation rate. Simulations indicate that doubling the ion number and optimization of laser cooling can help reduce the effect of losses. A range of kink dynamics is observed including configural change, motion and lifetime, and behavioral sensitivity to ion number.

An approach, differing from two commonly used methods (the stochastic Schrödinger equation and the master equation) but entrenched in the traditional density matrix formalism, is developed in a semiclassical setting in order to go from the solutions of the time-dependent Schrödinger equation to decohering and thermalized states. This is achieved by utilizing the time ergodicity, rather than the sampling (or ensemble) ergodicity, of physical systems. We introduce the formalism through a study of the Rabi model (a two-level system coupled to an oscillator) and show that our semiclassical version exhibits, both qualitatively and quantitatively, many features of state truncation and equilibration. We then study the time evolution of two qubits in interaction with a bosonic environment, such that the energy scale of one qubit is much larger and that of the other is much smaller than the environment's energy scale. The low-energy qubit decoheres to a mixture, while the high-energy qubit is protected through the adiabatic theorem. However, an interqubit coupling generates an overall decoherence and leads, for some values of the coupling, to long-term revivals in the state occupations.

Entanglement is known to be a relative notion, defined with respect to the choice of physical observables to be measured (i.e., the measurement setup used). This implies that, in general, the same state can be both separable and entangled for different measurement setups, but this does not exclude the existence of states which are separable (or entangled) for all possible setups. We show that for systems of bosonic particles there indeed exist such universally separable states: They are independently and identically distributed (i.i.d.) pure states. In contrast, there is no such state for fermionic systems with a few exceptional cases. We also find that none of the fermionic and bosonic systems admits universally entangled states.

We study an architecture for implementing adiabatic quantum computation with trapped neutral atoms. Ground-state atoms are dressed by laser fields in a manner conditional on the Rydberg blockade mechanism, thereby providing the requisite entangling interactions. As a benchmark, we study the performance of quantum annealing to the ground state of an Ising spin lattice. We model a proof-of-principle experiment in a realistic architecture, including details of the atomic implementation, with qubits encoded in the clock states of ${}^{133}$Cs. Numerical simulation yields fidelities >0.98 for up to four qubits, and implementations of 10–20 qubits are within the range of current technology.

We analyze various prominent quantum repeater protocols in the context of long-distance quantum key distribution. These protocols are the original quantum repeater proposal by Briegel, Dür, Cirac and Zoller, the so-called hybrid quantum repeater using optical coherent states dispersively interacting with atomic spin qubits, and the Duan-Lukin-Cirac-Zoller-type repeater using atomic ensembles together with linear optics and, in its most recent extension, heralded qubit amplifiers. For our analysis, we investigate the most important experimental parameters of every repeater component and find their minimally required values for obtaining a nonzero secret key. Additionally, we examine in detail the impact of device imperfections on the final secret key rate and on the optimal number of rounds of distillation when the entangled states are purified right after their initial distribution.

A bound-state field-theory approach to muonic hydrogen is set up using a variant of the Furry representation in which the lowest-order Hamiltonian describes a muon in the presence of a point Coulomb field, but the origin of the binding field is taken to be three charged quarks in the proton, which are modeled as Dirac particles that move freely within a spherical well. Bound-state field-theory techniques are used to evaluate one- and two-photon effects. Particular attention is paid to two-photon-exchange diagrams, which include the effect of proton polarizability. In addition, the modification of the electromagnetic self energy of the proton by the electric field of the muon is examined. Finally, the model is used to carry out a calculation of the static electric polarizability of the proton.

Based on the correlated ground-state wave function of an exactly solvable interacting one-dimensional two-electron model Hamiltonian we address the switch-off of the external confining interaction in order to calculate the exact time-evolving wave function from a correlated initial state. The Hartree-Fock approximation for the initial state of the model system is considered as well. By taking the differences of kinetic and potential energy terms obtained with these treatments, the time-dependent components of Wigner's correlation energy are determined.

We present a study into the collision-induced dissociation (possibly including electron stripping) of O${}_3^{+}$ and N${}_3^{+}$ with rest gas molecules (predominantly H${}_2$) in the heavy-ion storage ring CRYRING. The projectile ions had kinetic energies of 1.96 MeV (O${}_3^{+}$) and 2.25 MeV (N${}_3^{+}$) and from the experimental data we could derive the relative importance of the channels that produce at least one neutral product fragment. The dominant type of fragmentation for both ions involves the production of a single neutral fragment, namely an individual atom. We also find pronounced dissimilarities when comparing the O${}_3^{+}$ and N${}_3^{+}$ results, which we link to the stronger chemical bonds in the nitrogen system.

We study the vortex dynamics of a quantum-degenerate Bose gas through the intensity fluctuations of the interference from particles extracted at two different positions. It is shown numerically with classical field simulations that an interacting Bose gas with proliferating vortices exhibits long correlation times for these intensity fluctuations. This behavior is contrasted with the case of a noninteracting gas, which we describe analytically, and with the case of a well-condensed Bose gas without vortices. We discuss the observability of our predictions in quantum fluids of exciton-polaritons.

A method for performing nonlocal interferometry using phase-entangled macroscopic coherent states is described. The required entanglement can be generated using weak nonlinearities while Bell's inequality can be violated using single photons as a probe. The entanglement is relatively robust against photon loss and Bell's inequality can be violated over a relatively large distance in optical fibers despite the fact that a large number of photons are absorbed in the process.

We show that classical light diffraction in arrays of specially modulated coupled optical waveguides can simulate the quantum process of two-mode squeezing in nonlinear media, with the waveguide mode amplitudes corresponding the signal and idler photon numbers. The whole Fock space is mapped by a set of arrays, where each array represents the states with a fixed difference between the signal and idler photon numbers. We demonstrate a transition from photon number growth to Bloch oscillations with periodical revivals of an arbitrary input state, associated with an increase of the effective phase mismatch between the pump and the squeezed photons.

We have experimentally studied the formation of ultracold RbCs molecules by photoassociation at short internuclear range followed by spontaneous emission. The observation of new excited levels with respect to our previous work [Gabbanini and Dulieu, Phys. Chem. Chem. Phys. 13, 18905 (2011)] on both sides of the $\text{Rb}\left(5S_{1/2}\right)+\text{Cs}\left(6P_{3/2}\right)$ dissociation limit gives information on the state responsible for molecule formation. The molecules are mainly formed in deeply bound levels of the electronic triplet ground state. The observed photoassociation transitions present differences in terms of saturation.

It is well known that classical information can be cloned, but nonorthogonal quantum states cannot be cloned, and noncommuting quantum states cannot be broadcast. We conceive a scenario in which the object we want to broadcast is the statistical distinguishability, as quantified by quantum Fisher information, about a signal parameter encoded in quantum states. We show that quantum Fisher information cannot be cloned, while it might be broadcast even when the input states are noncommuting. This situation interpolates between cloning of classical information and no broadcasting of quantum information, and indicates a hybrid way of information broadcasting which is of particular significance from both practical and theoretical perspectives.